1. Field of the Invention
The present invention relates to an optical system for detecting focusing condition of a camera, and specifically relates to such optical system to be usable for an autofocus apparatus for single-lens reflex cameras and video cameras.
2. Description of the Prior Art
Recently, various kinds of optical systems for focus detection by a phase difference method which are used for cameras with auto-focusing function and the like have been suggested. According to the phase difference method, detection of focusing condition is established by detecting the relative positions of a plurality of re-formed images.
FIG. 5 shows a schematic configuration of a conventional optical system for detecting focusing condition by phase difference method. In this optical system, an aperture mask A.sub.M, a pair of image reforming lenses L.sub.1 and L.sub.2, and a line sensor P.sub.0 are disposed in sequence behind a condenser lens L.sub.0. A pair of those image re-forming lenses L.sub.1 and L.sub.2 are disposed nearly symmetrically with respect to an optical axis l (hereinafter referred to as a principal optical axis) of a taking lens, and re-form an image formed by the taking lens as a first image and a second image in cooperation with the condenser lens L.sub.0. Here, the distance between the first image and the second image which are re-formed by the image re-forming lenses L.sub.1 and L.sub.2 is varied depending on the focusing condition of the taking lens. A first and a second photo-detector arrays I and II of the line sensor P.sub.0 are disposed respectively in one line at or in the vicinity of a position which is conjugate with a predetermined image forming plane FP of the taking lens with respect to the condenser lens L.sub.0 and a pair of the image re-forming lenses L.sub.1 and L.sub.2. The focusing condition of the taking lens can be detected by detecting the positions of the first image and the second image on the first and the second photo-detector arrays I and II.
In FIG. 5, assuming that the magnification of image of the optical system is constant, a length S of the focusing condition detecting area on the predetermined image forming plane FP of the taking lens is determined by lengths S.sub.I and S.sub.II of the first and the second photo-detector arrays I and II of the line sensor P.sub.0. Accordingly, in order to lengthen the length S of the focusing condition detecting area, the lengths S.sub.I and S.sub.II of the first and the second photo-detector arrays I and II are required to be extended.
FIG. 7 is a schematic configuration view of an optical system for detecting focusing condition in the case where the length S of the focusing condition detecting area is lengthened. In accordance with elongation of the lengths S.sub.I and S.sub.II of the first and the second photo-detector arrays I and II, the distance between the first image and the second image to be re-formed is required to be extended. In the optical system shown in FIG. 7, by extending an distance l between centers O.sub.1 and O.sub.2 of curvature of a pair of the image re-forming lenses L.sub.1 and L.sub.2 in comparison with the optical system shown in FIG. 5, the optical path for forming the first image and the second image is deflected from the optical path as shown by broken lines to the optical path as shown by alternate long and two short dashes lines and thereby the distance between the first image and the second image is extended. Here, the distance between a pair of diaphragm apertures A.sub.1 and A.sub.2 can also be extended in addition to that between centers O.sub.1 and O.sub.2 of curvature, but the light flux for detecting the focusing condition becomes easy to be eclipsed, and the F number of the interchangeable lens capable of detecting the focusing condition is limited, and therefore, here, the distance between the image re-forming lenses L.sub.1 and L.sub.2 is extended from the distance as shown in FIG. 5 to that as shown in FIG. 7 in relation to the diaphragm apertures A.sub.1 and A.sub.2, and thereby the distance between the first image and the second image is extended.
In the Japanese Laid-Open Patent Publication No. 78518/1987, it is proposed that a reflecting member having at least two reflecting mirrors is disposed behind each image re-forming lens, and thereby the first image and the second image are separated in the direction of disposition of the photo-detector arrays. However, such disposition of the reflecting member for each image re-forming lens results in raising a problem that the configuration is complicated and the size becomes larger. It is therefore preferable that the focusing condition detecting apparatus for a camera required to be smaller in size utilizes the eccentricity of the image re-forming lenses.
However, where the extension of the focusing condition detecting area is established by the eccentricity of the image re-forming lenses L.sub.1 and L.sub.2, a problem exists that the field curvatures of the first image and the second image to be re-formed appear greatly, and the magnitudes of point images on the photo-detector arrays I and II becomes asymmetric with respect to center points of the photo-detector arrays I and II, respectively, resulting in a focusing condition detection error.
FIG. 6 and FIG. 8 show the magnitudes of the point images on the photo-detector arrays I and II shown in FIG. 5 and FIG. 7, respectively. As shown in FIG. 5, in the case where the distance l between the center axes l.sub.1 and l.sub.2 of curvature of the image re-forming lenses L.sub.1 and L.sub.2 is determined so that light flux from an intersection point C of the predetermined image forming plane FP and principal optical axis l.sub.0 travels nearly straight and the image of the point C is re-formed at center points C.sub.I and C.sub.II of the first and the second photo-detector arrays I and II, respectively, as shown in FIG. 6, no significant difference is produced between the magnitudes of point images of end points A and B of the focusing condition detecting area which are re-formed at end points A.sub.I, A.sub.II and B.sub.I, B.sub.II on the photo-detector arrays I and II, respectively.
On the other hand, as shown in FIG. 7, in the case where the distance l between the centers of curvature of the image re-forming lenses L.sub.1 and L.sub.2 is extended so that light flux from the intersection point C is deflected by the image re-forming lenses L.sub.1 and L.sub.2 and the image of the point C is re-formed at the center points C.sub.I and C.sub.II of the photo-detector arrays I and II, respectively, the light flux incident on the photo-detector array I from, for example, a point A in the focusing condition detecting area is not deflected so much when passing through the image re-forming lens L.sub.1, but it is deflected considerably when passing through the image re-forming lens L.sub.2, and therefore it is focused before the point where the light flux passing through the image re-forming lens L.sub.1 is focused, due to an influence of comatic aberration. This means that the point image re-formed at the end point A.sub.II of the photo-detector array II becomes larger than the point image re-formed at the end point A.sub.1 of the photo-detector array I. Accordingly, as shown in FIG. 8, a significant difference is produced between the magnitudes of the point images of the end points A and B of the focusing condition detecting area which are re-formed at the end points A.sub.I, A.sub.II and B.sub.I, B.sub.II, respectively of the photo-detector arrays I and II, and magnitudes of the point images on the photo-detector arrays I and II are asymmetric with respect to the center points C.sub.I and C.sub.II, respectively. Then, this causes a detection error of focusing condition.
FIG. 10 shows outputs of the photo-detector arrays I and II when a subject of white stripes on a black ground is put so that, as shown in FIG. 9, a white stripe (a) comes near the end point A of the focusing condition detecting area F.sub.A, a white stripe (b) comes near the end point B and a white stripe (c) comes in the vicinity of the center point C, respectively. In this case, widths of the images of the stripe (c) which are respectively re-formed in the vicinity of the center points C.sub.I and C.sub.II of the photo-detector arrays I and II are same, but widths of the images of the stripe (a) which are respectively re-formed in the vicinity of end points A.sub.I and A.sub.II are different, and widths of the images of the stripe (b) which are respectively re-formed in the vicinity of end points B.sub.I and B.sub.II are also different. Thus, when the magnitudes of the point images at the corresponding view points of the photo-detector arrays I and II differ remarkably, the degree of identity of the first image and the second image is reduced, resulting in a deterioration of accuracy in focusing condition detection. Furthermore, this phenomenon becomes greater as the image point shifts farther from the principal optical axis l.sub.0, and therefore, the merit of extending the focusing condition detecting area cannot be fully obtained.
In addition, in the Japanese Laid-Open Patent Publication No. 79407/1987, reference is made on the conditions of the thickness of the image re-forming lens, but it does not disclose the conditions for correcting the magnitudes of the point images to be re-formed, and the configuration of the image re-forming lens shown in this prior art is a combination of a prism surface and a convex lens surface, and therefore this does not serve to solve the above-described technical problem.